Fuel cells, devices transforming chemical energy directly into electricity, have been regarded as one of the promising clean future power sources. Proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) using polymeric proton conductive membranes as one of key components are drawing more and more attention for their utility in automotive and portable electronic applications.1,2 A continuous effort is being made to develop new high-performance proton conductive membranes as an alternative to Nafion®, which is the principal material used as the polymeric electrolyte in PEMFC systems because of its excellent chemical and mechanical stabilities, and high proton conductivity. However, high cost, low operation temperature (<80° C.), high methanol crossover, and environmental recycling uncertainties of Nafion and other similar perfluorinated membranes are limiting their widespread commercial application in PEMFC and DMFC.3-6 
Aromatic polymers with sulfonic acid groups may be promising materials for PEMs because of their outstanding thermal and chemical stability. Sulfonated derivatives of polyethersulfone (SPES),7-9 polyimide (SPI),10-12 polyimidazole,13 polyarylether,14,15 polyphenylene16,17 and poly(phenylquinoxaline)18 are among those being investigated as potential PEMs. However, to further simplify the preparation process and enhance performance of these materials, there are still some challenges, such as stereo-controllable chemical structures including easily-controllable degree of sulfonation (DS) and sulfonation sites, and well refined microstructure by grafting, alternating and blocking polymerization.19 
Aromatic polyetherketones (aromatic PEKS) are well known as high performance thermoplastics for their overall combination of chemical, physical and mechanical properties.20,21 As a class of promising PEM materials, several series of PEKs have been prepared so far. Most of the sulfonated PEK membranes were developed based on post-sulfonated commercial polymers or on copolymers produced from sulfonated monomers. Two general synthetic approaches have been used to realize the introduction of sulfonic acid groups into this family of polymers.3 One approach is post-sulfonation of existing polymers, and the other is direct copolymerization of sulfonated monomers. In the case of the post-sulfonation approach, this method is attractive because of the available resource of commercial polymers, such as Victrex® poly(ether ether ketone) (PEEK), as well as simple reaction procedures, enabling the process to be readily scaled up. Meanwhile, difficulties may occasionally be encountered in the precise control of the sulfonation sites and the degree of sulfonation (DS), resulting in a random and less defined distribution of sulfonic acid groups along the polymer chain. In addition, rigorous reaction conditions, such as high temperature and strongly acidic sulfonating agent, are usually used to prepare sulfonation polymers, which in some cases may lead to the occurrence of side reactions and degradation of the polymer backbone.3,19-25 In the case of the sulfonated monomer approach, despite the limited number of available sulfonated monomers and the preparation difficulties of the some sulfonated monomers, the direct copolymerization of sulfonated monomer with other nonsulfonated monomers has the potential for synthesizing random copolymers with a better control of sulfonation content (SC) and more defined chain structures in comparison with the copolymers by post-sulfonation method.26-28 Therefore, it is of interest to prepare sulfonated polymers via a post-sulfonation method that provides for controllable sulfonation sites and DS in a similar way to polymers prepared by the direct copolymerization methods.
Recently, there have been several stereo-controlled sulfonated polymers reported by Miyatake and Hay et al., and they also suggested that the polymers with the sulfonated groups attached to pendent side groups are very stable under heat, hydrolysis and oxidation.29,30 Jannasch et al. reported several side-chain-acid poly(ether sulfone)s with some attractive properties based on lithiation reaction.24,31 Meanwhile, polyimides with side-chain-acid substituents were also developed by Okamoto et al and Watanabe et al.32-34 
There remains a need for effective materials for proton conducting membranes and for controllable methods of preparing such materials.